The present invention is directed to depositing ceramic films on a substrate, and more particularly to a method and apparatus for depositing ceramic films by vacuum arc deposition.
Vacuum (or cathodic) arc deposition is a method for depositing a thin film from a solid source (the consumable cathode) with a very high growth rate (References 1 and 2). In some forms, several solid sources may be used to stoichiometrically mix the corresponding species leading to compound films (References 3 and 6). The vacuum arc can also be run in a background gas for reactive deposition of a compound film, e.g., TiN and CrN for coating steels (References 7 and 8). Vacuum arc deposition has in general been limited to metal cathodes, which typically sustain discharge current densities above 1MA/cm2 in non-stationary spots several xcexcm in diameter (Reference 9). One or more spots may be sustained (xcx9c50A per spot). At these cathodic spots, cathode material is efficiently converted to fully ionized plasma, which can be guided (e.g. magnetically) to a substrate for deposition. This process, however, has the potential of creating tremendous, localized stresses in the cathode material, especially if the thermal and electric conductivities are low, as they will be in a non-metal. This would clearly be the case for, for example, a boron carbide (B4C) cathode, typically produced via pressed powder metallurgy.
Recently, there has been some research activity in the development of vacuum arc deposition from non-metal (ceramic or semiconducting) cathodes. Researchers in England and Israel have independently succeeded in the operation of a continuous vacuum arc on a polycrystalline silicon cathode and have deposited a-Si films of near-electronic quality (References 10-13). Both groups, however, reported that problems remained regarding cathode fracturing. A group in Germany made the first published successful run of a vacuum arc on a pure boron cathode (References 14-15). The process was run reactively in a nitrogen background to make c-BN or in an Ar background because of problems running in vacuum in a stable fashion.
Encouraged by these developments, the inventors of the present invention pursued research to develop vacuum arc deposition of boron and boron carbide. Both of these materials make films that are of great interest in Fusion as well as in non-Fusion applications. Developments in the processing and handling of the cathodes, before and during the deposition, have allowed the inventors of the present invention to demonstrate continuous vacuum arc operation on both boron and boron carbide cathodes in a true vacuum mode ( less than 1 mPa). The present invention is believed to be the first successful deposition of stoichiometrically correct boron carbide from a boron carbide cathode. Although this technology can be potentially applied to other non-metals (as long as powders are available to consolidate into cathodes), this development was motivated by a need for improved coating methods for radio-frequency (rf) antennas used in magnetic fusion energy (MFE). Boron carbide is a material traditionally used in this application, primarily because of the low atomic number of the materials in this compound.
The recent experience from tests of rf antennas in tokamak experiments at Tore Supra and DIII-D has been that B4C is a desirable coating, but needs to be thick (due to erosion) and yet have good thermal conductivity, so that the underlying cooling channels can rapidly remove the heat from the surface of the coating (Reference 16). Unfortunately, the commercial sources for appropriate boron carbide coating are very limited and fall between two extremes. Plasma sprayed coatings can be made quite thick ( greater than 10 xcexcm). However their inherent porosity substantially limits the thermal conductivity. On the other hand, sputtered boron carbide generally produces dense films that are limited in the thickness due to the high cost and low deposition rates, as well as to stresses in the film.
Vacuum arc deposition offers the possibility of dense films, deposited at a lower cost and much higher rate. Also, independent substrate bias and heating can lead to deposition of oriented crystalline film with good thermal conductivity in the direction of crystalinity. As mentioned, this invention entails the first successful deposition of boron carbide using vacuum arc technology. However, the films were produced without biasing and were amorphous. The stoichiometry of the deposited film matched that of the B4C cathode. Heating up to 600xc2x0 C. did not produce any crystallinity, but is believed to have helped in making films as thick as 350 nm without significant stresses.
Despite the absence of crystallinity, the coatings obtained by the present invention are attractive for fusion applications beyond the antennas. As a boron-rich, low-Z material, B4C would be attractive as a first wall coating in a fusion reactor. Like any other carbonaceous material, one might expect some co-deposition of sputtered material with potential retention of radioactive tritium in a reactor environment (Reference 17). However, contrary to pure carbon materials, B4C has much lower chemical and high-temperature sputtering rates, and has the added benefits of oxygen gettering due to the large boron content (Reference 18). The low carbon content suggests lower hydrogen isotope retention. Of course, for an application to relatively smaller structures, such as the antennas, this issue is even less of a concern.
Finally, one can benefit from the good vacuum compatibility of the vacuum arc process by considering using the method of the present invention for in-situ repair of in-vessel components in a fusion reactor or a reactor-relevant experiment, such as ITER (References 19-21).
Outside fusion, boron carbide is a valuable metallurgical coating. It is particularly desirable in its amorphous phase, as obtained by the present invention, because of the combination of high hardness and low friction coefficient. A good example application would be as a coating for bearings or for cutting tools.
Various methods and apparatus for depositing coatings are disclosed in U.S. Pat. Nos. 3,944,683; 4,452,686; 4,496,868; 4,551,221; 4,645,895; 4,716,083; 5,072,992; 5,306,408; 5,582,874; 5,635,254; 5,896,012; 5,962,288; 6,054,187; 6,087,069; 6,120,640 and H. Shinno, T. Tanabe, M. Fujitsuka and Y. Sakai, xe2x80x9cCharacterization of Carbon-Boron Coatings Prepared on Molybdenum by a Vacuum Arc Deposition Methodxe2x80x9d, Thin Solid Films 189 (1990) 149-159. However, conventional techniques do not deposit a stoichiometrically correct coating of a ceramic material, such as boron carbide, titanium diboride, and lanthanum hexaboride.
The principal object of the present invention is to provide a method and apparatus for depositing ceramic films by vacuum arc deposition which overcome the drawbacks associated with conventional techniques.
An object of the present invention is to provide a method and apparatus for depositing ceramic films by vacuum arc deposition which assure long-term survival of the cathode and reduce the production of macroparticlaes that negatively impact the quality of the deposited films.
Another object of the present invention is to provide a method and apparatus for depositing ceramic films by vacuum deposition arc which produce thick films and yet have good thermal conductivity.
Yet another object of the present invention is to provide a method and apparatus for depositing ceramic films by vacuum arc deposition which produce dense films that are deposited at a lower cost and a much higher rate than conventional techniques and devices.
An additional object of the present invention is to provide a method and apparatus for depositing ceramic films by vacuum arc deposition which deposits a stoichiometrically correct coating of an electrically conductive ceramic material, such as boron, boron carbide, lanthanum hexaboride, etc., on a substrate.
Yet an additional object of the present invention is to provide a new method for coating a workpiece with boron carbide or other chemically related, ceramic coatings.
In summary, the main object of the present invention is to provide a method and apparatus for rapid coating of a surface by vacuum arc evaporation of a conductive, consolidated compound ceramic cathode. The resulting coating has nearly the same stoichiometry as the source material.
In accordance with the present invention, a method of depositing a ceramic film on a substrate by vacuum arc deposition, includes providing a vacuum chamber including a cathode comprised of a ceramic material to be deposited on a substrate, and an anode positioned downstream of the cathode, providing a substrate, preheating the cathode to a predetermined temperature, triggering an arc at the cathode surface to generate plasma for flowing towards the anode, guiding the plasma through the anode and towards the substrate, and impinging the plasma onto the substrate to deposit a substantially stoichiometric film of the ceramic material thereon.
In accordance with the present invention, an apparatus for depositing a ceramic film on a substrate by vacuum arc deposition, includes a vacuum chamber, a cathode comprised of an electrically conductive ceramic material to be deposited on a substrate, an electrically insulating member about the cathode, a heater for preheating the cathode to a predetermined temperature, an anode positioned downstream of the cathode and including an opening to allow ions of the ceramic material from the cathode to flow therethrough, a substrate support positioned downstream of the anode, and a plurality of magnetic members disposed around the vacuum chamber for guiding the ions from the cathode in a predetermined direction.
The novel method and apparatus of the present invention differ from the conventional vacuum-arc deposition techniques in a number of respects: (1) While the usual application of the vacuum arc as a metallurgical coater is to apply metal coatings or add metals to coatings by evaporating a metal cathode, a non-metal (ceramic) cathode is used in this invention, (2) while a conductive ceramic cathode made by consolidation of powders can in principle be used as the cathode of a vacuum arc, heating of the cathode is used in the present technique to further reduce the electrical resistance and to relieve the large stresses of the arc, which otherwise lead to fracture of the cathode, (3) since, even with heating, cathodes of these materials have been found to break under the stressful conditions of the arc, the technique of the invention uses cathodes consolidated with microwave energy to improve the integrity of the material, (4) to avoid any further stresses to the cathode, a soft-triggering method is used which is located at the center of the cathode to assure the initiation of the discharge at that location, and (5) a non-conductive ceramic shield is used around the cathode to help contain the electrical discharge within the extent of the cathode and to help guide the resulting ionized vapor toward the deposition region.